1. Field of the Invention
The present invention relates to a combustion control device and, more particularly, to a combustion control device of this kind which is adapted to detect flames from a combustion means mounted in a combustion chamber and control the combustion of the combustion means in accordance with a flame detection signal from a flame detecting means.
2. Description of the Prior Art
Conventional combustion control devices employ an ultraviolet ray detecting tube for detecting the ultraviolet rays emitted from flames as a flame detecting means.
FIGS. 1 and 2 are longitudinal sectional views respectively showing the above-mentioned ultraviolet ray detecting tube. The ultraviolet ray detecting tube shown in FIG. 1 is a glass tube 31 filled with a specific gas, in which a cathode 32 and an anode 33 are provided. Across these two electrodes is applied a voltage from an external power source (not illustrated).
When ultraviolet rays generated from flames are emitted, during the application of the voltage, photoelectrons released from the cathode 32 collide against gas molecules in the glass tube 31, repeating ionization to redouble the amount of gas. Thus the discharge current flows from the cathode 32 to the anode 33, thereby detecting the presence of flames.
An ultraviolet ray detecting tube shown in FIG. 2 is a glass tube 31 filled with a specific gas, in which the cathode 32 and the anode 33 are contained. The cathode 32 and the anode 33 are both provided with a flat electrode plate 34 and a reticulate electrode plate 35 which are disposed opposite to each other. The voltage is applied across the cathode 32 and the anode 33; flame detection is done in a similar manner as the ultraviolet ray detecting tube of FIG. 1.
The combustion control device of prior art constructed as described above has such a problem that when a self-discharge of electricity is caused by a fault or deterioration of the ultraviolet ray detecting tube, a signal indicating the presence of flames is generated even if flames do not exit and therefore no ultraviolet rays are detected (pseudo flame state), thus presenting a trouble. Specifically, since the self-discharge of the ultraviolet ray detecting tube cannot be distinguished from the discharge caused by the presence of flame, it is impossible to find out such defective condition of the tube. Conventionally, the combustion control device is safely stopped when the tube is detected to be broken, thereby presenting a problem that the above-mentioned defective tube is overlooked and cannot be found.